docs: def _get_model_fitting(self, con_est_id):
          """
          Retreive model fitting that corresponds to contrast with identifier
          'con_id'
          from the list of model fitting objects stored in self.model_fittings
          """
          for (mpe_id, pe_ids), contrasts in self.contrasts.items():
              for contrast in contrasts:
                  if contrast.estimation.id == con_est_id:
                      model_fitting_id = mpe_id
                      pe_map_ids = pe_ids
                      break
  
          for model_fitting in self.model_fittings:
              if model_fitting.activity.id == model_fitting_id:
                  return (model_fitting, pe_map_ids)
  
          raise Exception("Model fitting of contrast : " + str(con_est_id) +
                          " not found.")